Combine harvesters harvest cereal grain crops, such as wheat, oats, rye, barley, corn, soybeans and flax. Grain and straw are separated in a combine harvester. Following the separation process, waste straw and chaff is supplied to a chopper for shredding and distributing back over the field in an even spread pattern.
During the harvesting process weed seeds and grain seed are discharged with the residue into the chopper and spread back onto the field. The combine is then effectively acting as a seeder to evenly spread the seed back onto the field. In a number of areas of the world herbicides are used heavily to control the weed seeds however this has led to weed and grain seed that has become resistant to the herbicide.
It is known that if the seed can be removed or destroyed before the combine spreads it back onto the field the cycle can be stopped. Research has shown that, with three consecutive cycles of weed and grain removal, significant reductions in herbicide can be obtained providing huge saving for farmers.
One recent approach is shown in WO 2014/127408 published Aug. 28, 2014 and assigned to Grains Research Development Corporation Australia which shows that a plurality of impacts at relatively high speed of the seeds with a stationary object causes breakdown of the seed sufficient to prevent germination. Thus they have developed a cage mill which is integrally mounted inside the combine harvester so as to receive waste material (discarded seeds and chaff) from the sieve. The cage mill assembly includes at least one rotating ring carrying a plurality of blades and a series of outer stationary rings or fixed blades. Thus the seeds are accelerated outwardly by escaping centrifugally from the rotating blades into the surrounding stationary blades of the outer rings where a series of impacts occur as the seeds move outwardly into and through the fixed blades. The seeds are released outwardly under the centrifugal force from the stationary blades and escape outwardly into a peripheral channel for discharge.
The document shows evidence that four impacts at relatively high speed are sufficient to cause the required breakdown of the seeds, for example to obtain a 95% kill rate.
However the cage mill shown is large and complex with numerous rings running in opposite directions. Should a rock, or other hard material enter the mill, the entire cage mill would need to be replaced. Thus the system may function to destroy the seeds but has practical difficulties as it is without consideration of other obstacles passing through the assembly. The assembly runs at a very high rotational speed, so the precision in manufacturing is critical. Although this is believed that this arrangement is closer to commercialization a number of problems remain with the design.
U.S. Pat. No. 3,448,933 (Roy) issued Jun. 10, 1969 describes a cone style grinding shear mill used to process weed seed. All excess chaff and weed seed is processed by the unit. However it is a permanently fixed grinder without a means to bypass material other than residue. It would also allow passage of small fine seeds as it would need to be set to the average seed size to allow adequate throughput.
U.S. Pat. No. 5,059,154 (Reyenga) issued Oct. 22, 1991 discloses a pair of rollers to mill seeds smaller than grain that are in the clean grain auger. This does not address seeds thrown over the back of the sieve and would not work if placed behind the sieve as today's combines the chaff stream is often 6 inches thick which would cushion the seeds and allow the spread of live seed back onto the field.
In AU Published Application 2001/038781 an additional sieve is added to remove more of the chaff before milling, and separate the weed seed from the grain. However this is not practical with today's combines. All combines throw out some grain and farmers want the herbicide tolerant grain removed as well.
U.S. Pat. No. 8,152,610 (Harrington) issued Apr. 10, 2012 discloses an arrangement which processes all the chaff coming off of the sieves and blows it to a trailing cart to pulverize all of the residue. The cart requires a second engine running in the dust of the combine and the mill requires a significant amount of power to pulverize and discharge the residue back onto the field. The cage mill disclosed is large and complex with numerous rings running in opposite directions. Again, the rings have not removable parts so should a rock, or other hard material enter it the entire cage mill would need to be replaced. The cost of this system will limit its commercial viability.
The term weed seed destruction used herein is used somewhat colloquially in that the seeds are not annihilated but are devitalized or rendered so that they cannot germinate. It will of course also be appreciated that not necessarily each and every seed is destroyed but that the intention is that a significant number will be incapable of germination so as to reduce the number of emerging seeds in the growing season.